You need to have MATIC on Polygon. If you don’t have MATIC, you can get it by using one of these methods: https://pepperattack.com/faqs/how-to-get-matic-to-your-polygon-wallet/
Then swap MATIC to WETH using various channels.
- Open Metamask and choose Swap
- Select ETH from the dropdown list.
- Enter the amount to swap
- Review then Swap.
- Make sure your Metamask is connected to the Polygon network
- Choose the pair MATIC and WETH
- Enter the amount then Swap
You'll need to have ETH on Ethereum Mainnet.